Partitioning hydrologic contributions to an 'old-growth' riparian area in the Huron Mountains of Michigan, USA

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Over the past century, annual snowfall has increased across the 'snow-belt' region of the Upper Peninsula of Michigan, yet total annual precipitation has not changed, with potential impacts on hydrological processes and ecosystem composition. Using an integrated hydrochemical approach, we characterized groundwater discharge and quantified the contribution of snow- and rain-derived waters to groundwater for an old-growth riparian area within the Huron Mountains in northern Michigan. We then quantified the relative contribution of lateral, hillslope-derived groundwater and upstream lake water to streamwater, and the extent of hyporheic zone expansion and contraction during one growing season. During a period of above-average snowfall, yet below average growing season precipitation, ~80% of the riparian area's groundwater reservoir was derived from snowmelt. The relative contribution of groundwater to streamflow ranged from 70% in June to 100% in August. The remainder was derived from upstream lakes and wetlands, which dropped in elevation and relative contribution from June to August. Finally, the extent of the hyporheic zone was small ( < 50 cm from streambed surface) and contracted towards the stream during the recession limb of the hydrograph. We conclude that if snowfall continues to rise while total annual precipitation declines, in line with climate change scenarios for the region, then water fluxes from snowmelt will increasingly dominate summer baseflow and could potentially increase spring flooding. However, because of overall declines in precipitation, streamflow patterns will likely change towards lower overall flow following the influence of the snowmelt period. © John Wiley & Sons, Ltd.

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